Wireless communication systems are widely used to provide voice and data communication between multiple mobile stations or units, or between mobile units and stationary customer equipment. In a typical wireless communication system, such as a cellular system, one or more mobile stations or units communicate with a network of base stations linked at a telephone switching office. In the provision of cellular services within a cellular network, individual geographic areas or “cells” are serviced by one or more of the base stations. A typical base station includes a base station control unit and an antenna tower (not shown). The control unit comprises the base station electronics and is usually positioned within a ruggedized enclosure at, or near, the base of the tower. The control unit is coupled to the switching office through land lines or, alternatively, the signals might be transmitted or backhauled through backhaul antennas. A typical cellular network may comprise hundreds of base stations, thousands of mobile stations or units and one or more switching offices.
The switching office is the central coordinating element of the overall cellular network. It typically includes a cellular processor, a cellular switch and also provides the interface to the public switched telephone network (PSTN). Through the cellular network, a duplex radio communication link may be established between users of the cellular network.
In one typical arrangement of a base station, one or more passive antennas are supported at the tower top or on the tower and are oriented about the tower to define the desired beam sectors for the cell. A base station will typically have three or more RF antennas and possibly one or more microwave backhaul antennas associated with each wireless service provider using the base station. The passive RF antennas are coupled to the base station control unit through multiple RF coaxial cables that extend up the tower and provide transmission lines for the RF signals communicated between the passive RF antennas and the control unit during transmit (“down-link”) and receive (“up-link”) cycles.
The typical base station requires amplification of the RF signals being transmitted by the RF antenna. For this purpose, it has been conventional to use a large linear power amplifier within the control unit at the base of the tower or other support structure. The linear power amplifier must be cascaded into high power circuits to achieve the desired linearity at the higher output power. Typically, for such high power systems or amplifiers, additional high power combiners must be used at the antennas which add cost and complexity to the passive antenna design. The power losses experienced in the RF coaxial cables and through the power splitting at the tower top may necessitate increases in the power amplification to achieve the desired power output at the passive antennas, thereby reducing overall operating efficiency of the base station. It is not uncommon that almost half of the RF power delivered to the passive antennas is lost through the cable and power splitting losses.
More recently, active antennas, such as distributed active antennas, have been incorporated into base station designs to overcome the power loss problems encountered with passive antenna designs. Typical distributed active antennas include one or more sub-arrays or columns of antenna elements with each antenna element having a power amplifier provided at or near the antenna element or associated with each sub-array or column of antenna elements. The array of elements may be utilized to form a beam with a specific beam shape or multiple beams. One example of a distributed active antenna is fully disclosed in U.S. Ser. No. 09/846,790, filed May 1, 2001 and entitled Transmit/Receive Distributed Antenna Systems, which is commonly assigned with the present application and the disclosure of which is hereby incorporated herein by reference in its entirety.
The power amplifiers are provided in the distributed active antenna to eliminate the high amplifying power required in cellular base stations having passive antennas on the tower. By moving the transmit path amplification to the distributed active antennas on the tower, the significant cable losses and splitting losses associated with the passive antenna systems are overcome. Incorporating power amplifiers at the input to each antenna element or sub-array mitigates any losses incurred getting up the tower and therefore improves antenna system efficiency over passive antenna systems.
One problem encountered with distributed active antennas is that if one or more power amplifiers fail on the tower, the antenna elements associated with those failed power amplifiers become non-functional. This results in a loss of radiated power for the distributed active antenna and also a change in the shape of the beam or beams formed by the antenna array. Until the failed power amplifiers are repaired or replaced, the beam forming characteristics of the distributed active antenna are altered or, depending on the extent of the failure, the antenna becomes non-functional.
Therefore, there is a need for a distributed active antenna that is less susceptible to failure of the power amplifiers associated with the antenna elements in the transmit path.